PROJECT SUMMARY The long-term objective of this proposal is to reduce the risk of developing aggressive triple negative breast cancer (TNBC), especially for African-American women (AAW), by discovering how prolonged breastfeeding protects the breast from this risk. AAW have lower breastfeeding rates that likely contribute to the higher incidence of TNBC and higher mortality. Therefore, our work will specifically address this disparity in breast cancer outcomes faced by AAW. We developed unique mouse models for studying gradually involuting (GI- prolonged breastfeeding) vs abruptly involuting (AI-short breastfeeding) mammary glands. We have shown dramatic shifts in cellular composition of the mammary epithelial cell compartment and global changes in inflammatory markers and importantly have observed precancerous hyperplastic lesions within 120 days postpartum in the AI glands. These precancerous glands highly expressed the transcription factor Elf5, a key gene expressed by luminal progenitor cells, the cell-of-origin for TNBC. Based on these studies, we hypothesize that lack of breastfeeding not only alters the cellular composition and increases inflammation but leads to a higher risk of developing TNBC through Elf5 mediated aberrant differentiation: In this proposal, we will use innovative approaches including novel mouse models and patient samples to delineate the link between breastfeeding and TNBC. Aim 1A: Delineate the protective effects of GI vs. AI in mouse models of human breast cancer. 1B. determine if blocking estrogen signaling will abrogate the hyperplastic changes induced by AI. We will use MMTV-Cre;Brca1fl/fl;p53fl/fl mice and a chemical-carcinogen induced model to assess if AI accelerates tumor incidence and progression of TNBC. We will treat AI mice with tamoxifen to assess if progression to hyperplasia is mitigated. Aim 2: Elucidate the role of Elf5 and alterations in the microenvironment (ME) that led to the precancerous changes seen in the AI mammary glands. We will use mammary-specific Elf5 conditional knock-out mouse models and in vitro methods for this aim. Gene expression and CHIP-Seq will be utilized to identify the downstream effectors of Elf5. We will determine the differences in the ME between AI versus GI glands, especially the recruitment of different types of macrophages. Aim 3: Validate the histological and molecular changes observed in mice using human mammary tissue obtained from parous women who breastfed 0-3 (AI) vs. > 6 months (GI). We will obtain FFPE breast tissue from the Susan G. Komen for the Cure Tissue Bank to study the differences in collagen deposition and other inflammatory markers in the two cohorts. Racial differences will also be evaluated. Our work has a high translational significance in reducing the disparity in breast cancer related mortality among AAW by identifying novel preventive strategies for TNBC. Furthermore, these studies will lead to discovery of novel agents that could help women who are unable to breastfeed to reduce the risk of developing TNBC.